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class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="面试官-javascript中执行上下文和执行栈是什么"><a href="#面试官-javascript中执行上下文和执行栈是什么" class="header-anchor">#</a> 面试官：JavaScript中执行上下文和执行栈是什么？</h1> <p><img src="https://static.vue-js.com/8652b710-74c1-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h3 id="_4-执行上下文-执行环境"><a href="#_4-执行上下文-执行环境" class="header-anchor">#</a> 4. 执行上下文（执行环境）</h3> <ul><li><a href="https://juejin.im/post/6844903682283143181" target="_blank" rel="noopener noreferrer">理解 JavaScript 中的执行上下文和执行栈<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li> <li><a href="https://github.com/mqyqingfeng/Blog/issues/4" target="_blank" rel="noopener noreferrer">JavaScript深入之执行上下文栈<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul> <h4 id="_4-1-是什么"><a href="#_4-1-是什么" class="header-anchor">#</a> 4.1 是什么</h4> <p>概念：执行上下文（execution context）是评估和执行 JavaScript 代码环境的一个抽象概念。每当 Javascript代码在运行的时候，它都是在执行上下文中运行。</p> <p>解释：我们知道JavaScript 引擎是顺序执行的，但它是一段一段地分析执行的，当执行一段代码的时候，会进行一个“准备工作”，比如变量提升、函数提升等，这里的“准备工作”就叫做&quot;执行上下文(execution context)&quot;。</p> <h4 id="_4-2-类型"><a href="#_4-2-类型" class="header-anchor">#</a> 4.2 类型</h4> <p>JavaScript 中有三种执行上下文类型。</p> <ul><li><strong>全局执行上下文</strong> — 这是默认或者说基础的上下文，任何不在函数内部的代码都在全局上下文中。它会执行两件事：创建一个全局的 window 对象（浏览器的情况下），并且设置 this 的值等于这个全局对象。一个程序中只会有一个全局执行上下文。</li> <li><strong>函数执行上下文</strong> — 每当一个函数被调用时, 都会为该函数创建一个新的上下文。每个函数都有它自己的执行上下文，不过是在函数被调用时创建的。函数上下文可以有任意多个。每当一个新的执行上下文被创建，它会按定义的顺序（将在后文讨论）执行一系列步骤。</li> <li><strong>Eval 函数执行上下文</strong> — 执行在 <code>eval</code> 函数内部的代码也会有它属于自己的执行上下文，但由于 JavaScript 开发者并不经常使用 <code>eval</code>，所以在这里我不会讨论它。</li></ul> <h4 id="_4-3-执行栈"><a href="#_4-3-执行栈" class="header-anchor">#</a> 4.3 执行栈</h4> <p>也叫“调用栈”，是一种拥有 LIFO（后进先出）数据结构的栈，被用来存储代码运行时创建的所有执行上下文。</p> <p>当 JavaScript 引擎第一次执行脚本时，它会创建一个全局的执行上下文（<code>this</code> 绑定）并压入当前执行栈。每当引擎遇到一个函数调用，它都会为该函数创建一个新的执行上下文并压入栈的顶部。</p> <p>引擎会执行那些执行上下文位于栈顶的函数。当该函数执行结束时，执行上下文从栈中弹出，控制流程到达当前栈中的下一个上下文。</p> <h4 id="_4-4-三个重要属性"><a href="#_4-4-三个重要属性" class="header-anchor">#</a> 4.4 三个重要属性</h4> <ul><li>变量对象(Variable object，VO)</li> <li>作用域链(Scope chain)</li> <li>this</li></ul> <h3 id="_5-变量对象"><a href="#_5-变量对象" class="header-anchor">#</a> 5. 变量对象</h3> <p>概念：变量对象是与执行上下文相关的数据作用域，存储了在上下文中定义的变量和函数声明。</p> <ul><li>全局对象 - 全局执行上下文：是作用域链的头（在浏览器中，就是Window 对象）</li> <li>活动对象 - 函数执行上下文：是在进入函数上下文时刻被创建的，它通过函数的 arguments 属性初始化。arguments 属性值是 Arguments 对象。</li></ul> <h2 id="一、执行上下文"><a href="#一、执行上下文" class="header-anchor">#</a> 一、执行上下文</h2> <p>简单的来说，执行上下文是一种对<code>Javascript</code>代码执行环境的抽象概念，也就是说只要有<code>Javascript</code>代码运行，那么它就一定是运行在执行上下文中</p> <p>执行上下文的类型分为三种：</p> <ul><li>全局执行上下文：只有一个，浏览器中的全局对象就是 <code>window</code>对象，<code>this</code> 指向这个全局对象</li> <li>函数执行上下文：存在无数个，只有在函数被调用的时候才会被创建，每次调用函数都会创建一个新的执行上下文</li> <li>Eval 函数执行上下文： 指的是运行在 <code>eval</code> 函数中的代码，很少用而且不建议使用</li></ul> <p>下面给出全局上下文和函数上下文的例子：</p> <p><img src="https://static.vue-js.com/90dd3b60-74c1-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <p>紫色框住的部分为全局上下文，蓝色和橘色框起来的是不同的函数上下文。只有全局上下文（的变量）能被其他任何上下文访问</p> <p>可以有任意多个函数上下文，每次调用函数创建一个新的上下文，会创建一个私有作用域，函数内部声明的任何变量都不能在当前函数作用域外部直接访问</p> <h2 id="二、生命周期"><a href="#二、生命周期" class="header-anchor">#</a> 二、生命周期</h2> <p>执行上下文的生命周期包括三个阶段：创建阶段 → 执行阶段 → 回收阶段</p> <h3 id="创建阶段"><a href="#创建阶段" class="header-anchor">#</a> 创建阶段</h3> <p>创建阶段即当函数被调用，但未执行任何其内部代码之前</p> <p>创建阶段做了三件事：</p> <ul><li>确定 this 的值，也被称为 <code>This Binding</code></li> <li>LexicalEnvironment（词法环境） 组件被创建</li> <li>VariableEnvironment（变量环境） 组件被创建</li></ul> <p>伪代码如下：</p> <div class="language-js extra-class"><pre class="language-js"><code>ExecutionContext <span class="token operator">=</span> <span class="token punctuation">{</span>  
  ThisBinding <span class="token operator">=</span> <span class="token operator">&lt;</span><span class="token keyword">this</span> value<span class="token operator">&gt;</span><span class="token punctuation">,</span>     <span class="token comment">// 确定this </span>
  LexicalEnvironment <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>   <span class="token comment">// 词法环境</span>
  VariableEnvironment <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>  <span class="token comment">// 变量环境</span>
<span class="token punctuation">}</span>
</code></pre></div><h4 id="this-binding"><a href="#this-binding" class="header-anchor">#</a> This Binding</h4> <p>确定<code>this</code>的值我们前面讲到，<code>this</code>的值是在执行的时候才能确认，定义的时候不能确认</p> <h4 id="词法环境"><a href="#词法环境" class="header-anchor">#</a> 词法环境</h4> <p>词法环境有两个组成部分：</p> <ul><li><p>全局环境：是一个没有外部环境的词法环境，其外部环境引用为<code>null</code>，有一个全局对象，<code>this</code> 的值指向这个全局对象</p></li> <li><p>函数环境：用户在函数中定义的变量被存储在环境记录中，包含了<code>arguments</code> 对象，外部环境的引用可以是全局环境，也可以是包含内部函数的外部函数环境</p></li></ul> <p>伪代码如下：</p> <div class="language-js extra-class"><pre class="language-js"><code>GlobalExectionContext <span class="token operator">=</span> <span class="token punctuation">{</span>  <span class="token comment">// 全局执行上下文</span>
  LexicalEnvironment<span class="token operator">:</span> <span class="token punctuation">{</span>       <span class="token comment">// 词法环境</span>
    EnvironmentRecord<span class="token operator">:</span> <span class="token punctuation">{</span>     <span class="token comment">// 环境记录</span>
      Type<span class="token operator">:</span> <span class="token string">&quot;Object&quot;</span><span class="token punctuation">,</span>           <span class="token comment">// 全局环境</span>
      <span class="token comment">// 标识符绑定在这里 </span>
      outer<span class="token operator">:</span> <span class="token operator">&lt;</span><span class="token keyword">null</span><span class="token operator">&gt;</span>           <span class="token comment">// 对外部环境的引用</span>
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>

FunctionExectionContext <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token comment">// 函数执行上下文</span>
  LexicalEnvironment<span class="token operator">:</span> <span class="token punctuation">{</span>     <span class="token comment">// 词法环境</span>
    EnvironmentRecord<span class="token operator">:</span> <span class="token punctuation">{</span>    <span class="token comment">// 环境记录</span>
      Type<span class="token operator">:</span> <span class="token string">&quot;Declarative&quot;</span><span class="token punctuation">,</span>      <span class="token comment">// 函数环境</span>
      <span class="token comment">// 标识符绑定在这里      // 对外部环境的引用</span>
      outer<span class="token operator">:</span> <span class="token operator">&lt;</span>Global or outer <span class="token keyword">function</span> environment reference<span class="token operator">&gt;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>
</code></pre></div><h4 id="变量环境"><a href="#变量环境" class="header-anchor">#</a> 变量环境</h4> <p>变量环境也是一个词法环境，因此它具有上面定义的词法环境的所有属性</p> <p>在 ES6 中，词法环境和变量环境的区别在于前者用于存储函数声明和变量（ <code>let</code> 和 <code>const</code> ）绑定，而后者仅用于存储变量（ <code>var</code> ）绑定</p> <p>举个例子</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>  
<span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token number">30</span><span class="token punctuation">;</span>  
<span class="token keyword">var</span> c<span class="token punctuation">;</span>

<span class="token keyword">function</span> <span class="token function">multiply</span><span class="token punctuation">(</span><span class="token parameter">e<span class="token punctuation">,</span> f</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>  
 <span class="token keyword">var</span> g <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>  
 <span class="token keyword">return</span> e <span class="token operator">*</span> f <span class="token operator">*</span> g<span class="token punctuation">;</span>  
<span class="token punctuation">}</span>

c <span class="token operator">=</span> <span class="token function">multiply</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">,</span> <span class="token number">30</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>执行上下文如下：</p> <div class="language-js extra-class"><pre class="language-js"><code>GlobalExectionContext <span class="token operator">=</span> <span class="token punctuation">{</span>

  ThisBinding<span class="token operator">:</span> <span class="token operator">&lt;</span>Global Object<span class="token operator">&gt;</span><span class="token punctuation">,</span>

  LexicalEnvironment<span class="token operator">:</span> <span class="token punctuation">{</span>  <span class="token comment">// 词法环境</span>
    EnvironmentRecord<span class="token operator">:</span> <span class="token punctuation">{</span>  
      Type<span class="token operator">:</span> <span class="token string">&quot;Object&quot;</span><span class="token punctuation">,</span>  
      <span class="token comment">// 标识符绑定在这里  </span>
      a<span class="token operator">:</span> <span class="token operator">&lt;</span> uninitialized <span class="token operator">&gt;</span><span class="token punctuation">,</span>  
      b<span class="token operator">:</span> <span class="token operator">&lt;</span> uninitialized <span class="token operator">&gt;</span><span class="token punctuation">,</span>  
      multiply<span class="token operator">:</span> <span class="token operator">&lt;</span> func <span class="token operator">&gt;</span>  
    <span class="token punctuation">}</span>  
    outer<span class="token operator">:</span> <span class="token operator">&lt;</span><span class="token keyword">null</span><span class="token operator">&gt;</span>  
  <span class="token punctuation">}</span><span class="token punctuation">,</span>

  VariableEnvironment<span class="token operator">:</span> <span class="token punctuation">{</span>  <span class="token comment">// 变量环境</span>
    EnvironmentRecord<span class="token operator">:</span> <span class="token punctuation">{</span>  
      Type<span class="token operator">:</span> <span class="token string">&quot;Object&quot;</span><span class="token punctuation">,</span>  
      <span class="token comment">// 标识符绑定在这里  </span>
      c<span class="token operator">:</span> <span class="token keyword">undefined</span><span class="token punctuation">,</span>  
    <span class="token punctuation">}</span>  
    outer<span class="token operator">:</span> <span class="token operator">&lt;</span><span class="token keyword">null</span><span class="token operator">&gt;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>

FunctionExectionContext <span class="token operator">=</span> <span class="token punctuation">{</span>  
   
  ThisBinding<span class="token operator">:</span> <span class="token operator">&lt;</span>Global Object<span class="token operator">&gt;</span><span class="token punctuation">,</span>

  LexicalEnvironment<span class="token operator">:</span> <span class="token punctuation">{</span>  
    EnvironmentRecord<span class="token operator">:</span> <span class="token punctuation">{</span>  
      Type<span class="token operator">:</span> <span class="token string">&quot;Declarative&quot;</span><span class="token punctuation">,</span>  
      <span class="token comment">// 标识符绑定在这里  </span>
      Arguments<span class="token operator">:</span> <span class="token punctuation">{</span><span class="token number">0</span><span class="token operator">:</span> <span class="token number">20</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token operator">:</span> <span class="token number">30</span><span class="token punctuation">,</span> length<span class="token operator">:</span> <span class="token number">2</span><span class="token punctuation">}</span><span class="token punctuation">,</span>  
    <span class="token punctuation">}</span><span class="token punctuation">,</span>  
    outer<span class="token operator">:</span> <span class="token operator">&lt;</span>GlobalLexicalEnvironment<span class="token operator">&gt;</span>  
  <span class="token punctuation">}</span><span class="token punctuation">,</span>

  VariableEnvironment<span class="token operator">:</span> <span class="token punctuation">{</span>  
    EnvironmentRecord<span class="token operator">:</span> <span class="token punctuation">{</span>  
      Type<span class="token operator">:</span> <span class="token string">&quot;Declarative&quot;</span><span class="token punctuation">,</span>  
      <span class="token comment">// 标识符绑定在这里  </span>
      g<span class="token operator">:</span> <span class="token keyword">undefined</span>  
    <span class="token punctuation">}</span><span class="token punctuation">,</span>  
    outer<span class="token operator">:</span> <span class="token operator">&lt;</span>GlobalLexicalEnvironment<span class="token operator">&gt;</span>  
  <span class="token punctuation">}</span>  
<span class="token punctuation">}</span>
</code></pre></div><p>留意上面的代码，<code>let</code>和<code>const</code>定义的变量<code>a</code>和<code>b</code>在创建阶段没有被赋值，但<code>var</code>声明的变量从在创建阶段被赋值为<code>undefined</code></p> <p>这是因为，创建阶段，会在代码中扫描变量和函数声明，然后将函数声明存储在环境中</p> <p>但变量会被初始化为<code>undefined</code>(<code>var</code>声明的情况下)和保持<code>uninitialized</code>(未初始化状态)(使用<code>let</code>和<code>const</code>声明的情况下)</p> <p>这就是变量提升的实际原因</p> <h3 id="执行阶段"><a href="#执行阶段" class="header-anchor">#</a> 执行阶段</h3> <p>在这阶段，执行变量赋值、代码执行</p> <p>如果 <code>Javascript</code> 引擎在源代码中声明的实际位置找不到变量的值，那么将为其分配 <code>undefined</code> 值</p> <h3 id="回收阶段"><a href="#回收阶段" class="header-anchor">#</a> 回收阶段</h3> <p>执行上下文出栈等待虚拟机回收执行上下文</p> <h2 id="二、执行栈"><a href="#二、执行栈" class="header-anchor">#</a> 二、执行栈</h2> <p>执行栈，也叫调用栈，具有 LIFO（后进先出）结构，用于存储在代码执行期间创建的所有执行上下文</p> <p><img src="https://static.vue-js.com/9eda0310-74c1-11eb-ab90-d9ae814b240d.png" alt=""></p> <p>当<code>Javascript</code>引擎开始执行你第一行脚本代码的时候，它就会创建一个全局执行上下文然后将它压到执行栈中</p> <p>每当引擎碰到一个函数的时候，它就会创建一个函数执行上下文，然后将这个执行上下文压到执行栈中</p> <p>引擎会执行位于执行栈栈顶的执行上下文(一般是函数执行上下文)，当该函数执行结束后，对应的执行上下文就会被弹出，然后控制流程到达执行栈的下一个执行上下文</p> <p>举个例子：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token string">'Hello World!'</span><span class="token punctuation">;</span>
<span class="token keyword">function</span> <span class="token function">first</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Inside first function'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token function">second</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Again inside first function'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">second</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Inside second function'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token function">first</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'Inside Global Execution Context'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>转化成图的形式</p> <p><img src="https://static.vue-js.com/ac11a600-74c1-11eb-ab90-d9ae814b240d.png" alt=""></p> <p>简单分析一下流程：</p> <ul><li>创建全局上下文请压入执行栈</li> <li><code>first</code>函数被调用，创建函数执行上下文并压入栈</li> <li>执行<code>first</code>函数过程遇到<code>second</code>函数，再创建一个函数执行上下文并压入栈</li> <li><code>second</code>函数执行完毕，对应的函数执行上下文被推出执行栈，执行下一个执行上下文<code>first</code>函数</li> <li><code>first</code>函数执行完毕，对应的函数执行上下文也被推出栈中，然后执行全局上下文</li> <li>所有代码执行完毕，全局上下文也会被推出栈中，程序结束</li></ul> <h2 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h2> <ul><li>https://zhuanlan.zhihu.com/p/107552264</li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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